Final answer:
Chlorine has a higher ionization energy than aluminum because of its higher effective nuclear charge and its electron configuration that is closer to a full octet, making its valence electrons more difficult to remove.
Step-by-step explanation:
Chlorine has a higher ionization energy than aluminum because it has a greater effective nuclear charge on its valence electrons, due to its higher atomic number and the same shielding effect as aluminum. Both aluminum and chlorine are located in the third period of the periodic table, but chlorine has more protons, providing a stronger attraction to the electrons and making them harder to remove. Additionally, the electron configuration of chlorine ends with p⁵, which is closer to achieving a full octet and hence more stable, whereas aluminum's last electron is in the 3p¹ orbital, which is easier to remove due to a less stable electron configuration.
Reasons for different ionization energies include electron configurations, the effective nuclear charge, and the stability of resulting ions. For example, removing an electron from a stable configuration, such as those nearer a full octet, requires more energy. This explains why certain elements have unexpectedly high ionization energies compared to their neighbors in the periodic table. Ionization energy also increases with the degree of ion charge because the ion's electrostatic pull on the remaining electrons strengthens.